Printing, folding and assembly methods for forming 3D mesostructures in advanced materials

| A rapidly expanding area of research in materials science involves the development of routes to complex 3D structures with feature sizes in the mesoscopic range (that is, between tens of nanometres and hundreds of micrometres). A goal is to establish methods for controlling the properties of materials systems and the function of devices constructed with them, not only through chemistry and morphology, but also through 3D architectures. The resulting systems, sometimes referred to as metamaterials, offer engineered behaviours with optical, thermal, acoustic, mechanical and electronic properties that do not occur in the natural world. Impressive advances in 3D printing techniques represent some of the most broadly recognized developments in this field, but recent successes with strategies based on concepts in origami, kirigami and deterministic assembly provide additional, unique options in 3D design and high-performance materials. In this Review, we highlight the latest progress and trends in methods for fabricating 3D mesostructures, beginning with the development of advanced material inks for nozzle-based approaches to 3D printing and new schemes for 3D optical patterning. In subsequent sections, we summarize more recent methods based on folding, rolling and mechanical assembly, including their application with materials such as designer hydrogels, monocrystalline inorganic semiconductors and graphene. REVIEWS NATURE REVIEWS | MATERIALS VOLUME 2 | ARTICLE NUMBER 17019 | 1 © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. structures. For example, recent work demonstrates mechanical metamaterials40,42,43 in the form of ceramic and metallic microand nanolattices that reach previously inaccessible regimes of a parameter space defined by the strength, stiffness and weight. Advances in such types of mechanical metamaterials follow from the emergence of ingenious methods for multiphoton patterning and large-area projection micro-stereo lithography (LAPμSL)44 that provide access to hierarchical structures with disparate 3D features with length scales from nanometres to centimetres. Lightweight metamaterials with tunable 3D negative thermal expansion can be achieved with carefully designed 3D periodic lattice units containing constituents with varied thermal expansion coefficients49. This type of system can be realized with a multimaterial projection micro-stereolithography (PμSL) method in which two distinct beam constituents join to form composite microlattices49. Soft acoustic meta materials can be formed with 3D structures that exhibit strong low-frequency Mie resonances50–52. In certain examples, microfluidic strategies yield macroporous silicone microbeads randomly dispersed in gel matrices, with negative acoustic phase velocity over a broad range of frequencies50. 3D microand nanostructures also offer powerful engineering design options for microsystem technologies, with relevance to biomedical devices53–57, microelectromechanical systems (MEMS)58–60, energy storage platforms61–66, optoelectronic components67,68, and others69 (for example, electronics70–76) where conventional, planar architectures currently dominate. As with metamaterials, progress follows from the ability to realize 3D structures in advanced materials. In addition to the optical techniques described previously, innovations in 3D printing techniques77,78 and associated inks70,79–86 based on metal, metal oxides, biomaterials and biocompatible polymers represent notable recent developments. Alternative methods that exploit concepts in origami, kirigami87–92 and mechanically guided assembly74,75,93–97 provide complementary capabilities, with key advantages that derive from compatibility with modern planar technologies and associated thin-film deposition and processing techniques. This Review highlights the latest results and future trends in these types of techniques (that is, printing, folding, rolling and assembly) as routes to 3D mesostructures in advanced functional materials. The discussions begin with techniques for 3D printing that use scanning nozzles and patterns of light in schemes that utilize two-phonon lithography (TPL) and PμSL, including recent strategies that allow continuous, high-speed operation. Subsequent sections summarize methods based on folding and mechanical assembly, with application to materials such as designer hydrogels, monocrystalline inorganic semiconductors and graphene. We conclude by outlining some challenges and opportunities for future research. Nozzle-based 3D printing technologies The most widely used method for 3D printing relies on robotic control of scanning deposition nozzles in which 3D structures form in a layer-by-layer manner85,98–101. Materials typically formulated from particulate and/or polymeric species with desired rheological behaviour serve as ‘inks’. Depending on the properties of the inks and the nature of the delivery process, these techniques can be further classified into two groups85,98,102: inkjet printing for liquid inks and microextrusion printing for viscoelastic inks (FIG. 1a). Inkjet printing, also known as drop-on-demand printing, exploits nozzles that use thermal or acoustic processes to deliver liquid droplets to desired locations on the substrate85,98,102–104. By contrast, microextrusion printing uses pressure-induced flow to form continuous filaments of material, often designed to solidify immediately or shortly after emergence from the nozzle tip. In both cases, proper materials formulation is essential. For inkjet printing, the challenges are in realizing high solid content for thick deposits, but at sufficiently low viscosity to allow droplet generation in a manner that also avoids clogging of the nozzles105. Although inkjet printing has widespread applications in forming 2D structures, 3D architectures can be difficult to form, partly owing to the uncontrolled flow of inks as they impinge on a structured surface106. By comparison, microextrusion printing offers far greater possibilities in 3D structuring owing to the broad range of ink designs and feature sizes. Microextrusion printers that offer modest (slightly sub-millimetre) resolution are now widely available in the form of commercial, user-friendly tools. In most cases, a printing head robotically controls the extrusion of a material ink onto a substrate, whereby a handling and dispensing system pneumatically or mechanically (with a piston or screw) controls the extrusion of materials in well-defined ambient conditions85. Recent research establishes methods for fabricating sophisticated 3D mesostructures with microscale resolution in a broad range of materials, including concentrated colloidal107, nanoparticle108, fugitive83,109 and polyelectrolyte110 inks. Main areas of current interest are in conductive and biomaterial inks because of their relevance in key applications, from solid-state lighting and wearable devices to tissue scaffolds and regenerative medicine. The following subsections highlight some of the latest progress. Author addresses Center for Mechanics and Materials, AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China. Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA. Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA. Departments of Civil and Environmental Engineering, Mechanical Engineering, and Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA. Departments of Materials Science and Engineering, Biomedical Engineering, Neurological Surgery, Chemistry, Mechanical Engineering, Electrical Engineering and Computer Science; Center for Bio-Integrated Electronics; and Simpson Querrey Institute for BioNanotechnology, Northwestern University, Evanston, Illinois 60208, USA. R E V I E W S 2 | ARTICLE NUMBER 17019 | VOLUME 2 www.nature.com/natrevmats © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. 3D printing with conductive inks. One notable ad vance70,79,80 involves highly concentrated silver nanoparticle inks (>70 wt% Ag) for electrically conductive filaments in 3D arrangements (FIG. 1b,c). The visco elastic properties enable self-supporting structures, such as the arc-shaped features in FIG. 1b. Thermal annealing (for example, at 150–500 °C) performed after printing by convective heating70 or during printing by heating with a focused laser beam80 can substantially improve the conductivity and the mechanical properties70. As shown in FIG. 1d, 3D helical coils and butterfly shapes can be formed by heating during printing80. Electrically functional devices, such as 3D spanning interconnects in arrays of microscale light-emitting diodes (FIG. 1b) and hemispherical antennas (FIG. 1c), suggest broad utility in electronics and optoelectronics70,79. Other strategies use liquid-metal inks in the form of binary eutectic alloys of gallium and indium (EGaIn)81–83. EGaIn is a low-viscosity liquid at room temperature (with a melting point of ~15.7 °C). An ultrathin (~1 nm) passivating layer of gallium oxide forms immediately on exposure to air, thereby preventing spontaneous flow in ambient conditions81. As a result, at modest pressures (<5 kPa), metal filaments of EGaIn can be extruded to form free-standing structures. Additional options include the formation of 3D architectures of stacked droplets (FIG. 1e) delivered with a nozzle or of bars formed by the injection of metal into 3D microchannels created in sacrificial materials82,83. Sophisticated function in 3D structures typically demands the integration of several different types of materials, including, but not limited to, metals. Nature Reviews | Materials a 3D printing techniques b Silver interconnects in a LED array c Silver antenna on a hemispherical surface d 3D silver helices and butterfly shapes e 3D structures of liquid metal EGaIn f Quantum-dot LEDs Loading Inkjet Microextrusion LED components 3D printing process Switch